1. FIELD OF THE INVENTION
This invention relates generally to the silk screen printing art and more particularly to a novel printing screen stretch frame.
2. PRIOR ART
Simply stated, a printing screen stretch frame comprises a rectangular frame structure having a central rectangular opening and screen tensioning means along the sides of the frame opening for gripping edges of printing screen placed across one side of the frame and stretching the screen edgewise. One type of printing screen frame has screen tensioning draw bars which are movable laterally in the plane of the frame to stretch and release the printing screen. Another type of screen frame, referred to herein in places as a roller frame, has screen tensioning rollers which are rotatable to stretch and release the screen.
Two basic types of roller frames are known in the art. One type comprises four rigid frame members arranged along the four sides of a rectangle and joined end to end at the corners of the rectangle to form a rigid rectangular frame part and screen rollers rotatably mounted on the frame members. The other basic type of roller frame lacks such a rigid frame part and includes only corner brackets at the four corners of the frame, screen rollers extending along the four sides of the frame between the adjacent corner brackets, and roller mounting means securing the rollers to the corner brackets. This latter type of screen frame comprises only the corner brackets, rollers, and roller mounting means and is rigidized by rigidly securing the rollers to the brackets. The present invention is concerned with roller screen frames of this latter kind.
A number of roller screen frames with and without separate rigid frame members have been devised. Among these known frames are those illustrated in FIGS. 5-7 of the attached drawings and those described in the patents listed below.
The roller frame illustrated in FIGS. 5-7 of the attached drawings includes screen rollers 20 attached to corner brackets 22 by bolts 24 having outer heads 26 which are accessible for tightening and releasing the bolts and threaded shanks 28 which extend inwardly through the corner brackets, wrench heads 30 for turning the rollers, and end blocks 32 fixed within the roller ends into threaded engagement with nuts 34 fixed within the end blocks.
U.S. Pat. No. 3,601,912 dated Aug. 31, 1978, to Dubbs discloses a screen frame having screen rollers 14, 16 joined by corner brackets 18.
U.S. Pat. No. 3,908,293 dated Sep. 30, 1975 to Newman discloses a screen frame having screen rollers 18, 20 extending between corner brackets 26. Secured within the roller ends are end blocks 58 having integral wrench heads 86 for turning the rollers and integral threaded studs 72 which extend through the corner brackets into threaded engagement with outer lock nuts 84 for securing the rollers against rotation.
U.S. Pat. No. 4,430,814 dated Feb. 14, 1984, to Wulc discloses a screen frame having screen rollers 16, 18 extending between corner brackets 24 and secured to the brackets by threaded studs 34 which are press fitted within the roller ends and mount inner nuts 36 for turning the rollers and outer nuts 40 for securing the rollers against rotation.
U.S. Pat. No. 5,018,442 dated May 28, 1991, to Hamu discloses a screen frame having screen rollers 16 extending between corner brackets 14. Threaded studs 34, 34a are press fitted (FIG. 2) or threaded (FIG. 5) in end blocks 28, 28a fixed within the roller ends and mount inner nuts 38 for turning the rollers and outer nuts 40, 40a for securing the rollers against rotation.
The screen frames referred to above and other existing screen frames have certain deficiencies which this invention overcomes. The frame arrangement of FIGS. 5-7, for example, requires complex and costly machining of the end blocks 32 to fixedly receive the nuts 34. In the Dubbs, Hamu, and Wulc patents, tightening the lock nuts on the studs to secure the rollers against turning exerts substantial stress on the press-fitted or threaded joints between the studs and the rollers. In order to prevent their failure by rupture of the press-fitted joints and stripping of the threaded joints, the joints must have a relatively long axial length sufficient to resist such rupture or stripping. This, in turn, increases the mass of metal in and thereby the weight and cost of the rollers. Also, the threaded joints require internal threading of the end blocks and increases the frame cost. The use of cross bolts in Newman to secure the end blocks in the rollers requires machining of the rollers which increases the roller cost and renders the rollers prone to fluid leakage around the bolts into the rollers.